Electric power utility companies use a variety of techniques to meet fluctuating power demand while maintaining a relatively constant level of electric power production. One way to handle short lived, irregularly occurring electric power demand increases is to electrically couple an electric energy storage system to an electric power transmission system so that the energy storage system may be utilized, or turned on, to provide additional electric power during peak demand.
While some systems such as the one shown in U.S. Pat. No. 5,610,802 have been developed to address peak demand needs, ever increasing demand for power requires that such systems operate very efficiently, with little need for human supervision, be designed to minimize leaks or other releases of potentially hazardous materials, and be relatively inexpensive. Of particular importance to the efficient operation of such systems is the effective and efficient operation of the energy storing devices used in them. Often these devices are liquid electrolyte batteries, particularly zinc-bromine batteries.
Zinc-bromine batteries are a type of bipolar, metal-halogen battery with a stack of cells made from a series of alternating electrodes and separators. An electrolyte flows through the stack of cells and conducts electricity ionically. In a bipolar, zinc-bromine battery each cell includes a bipolar electrode upon which an anodic and a cathodic reaction occurs.
The electrolyte used in a zinc-bromine battery is a fluid containing aqueous zinc-bromide and quaternary ammonium salts. It is circulated through the cells to and from external reservoirs. For each cell, one half cell contains an anolyte and the other half cell contains a catholyte. The anolyte flows through a common anolyte manifold to each anodic half cell and the catholyte flows through a parallel common catholyte manifold to each cathodic half cell. The alternating separators and electrodes are sealed together in a manner that prevents communication between the anolyte and catholyte systems.
A zinc-bromine battery may be in various states including a charged state and a discharged state. In addition, the battery may cycle through these states. When a zinc-bromine battery is in a discharged state, the anolyte is chemically identical to the catholyte. When a zinc-bromine battery is charged the following reaction takes place. EQU Zn.sup.++ +2e.sup.-.fwdarw.Zn EQU 2Br.sup.-.fwdarw.Br.sub.2 +2e.sup.-
Zinc is plated on the anode, and bromine is evolved on the cathode. The bromine is immediately complexed by quaternary ammonium ions in the electrolyte to form a dense second phase that is removed from the battery stack with the flowing electrolyte. When the battery is charged, zinc is stored on one side of each electrode and complex bromine is stored in a catholyte reservoir.
During discharge, the following reaction takes place. EQU Br.sub.2 +2e.sup.- 2Br.sup.- EQU Zn .fwdarw.Zn.sup.++ +2e.sup.-
Zinc is oxidized and the released electrons pass through the electrode where they combine with molecular bromine to form bromide ions. Positively charged zinc ions travel through the separator and remain in solution, and at the same time, bromide ions pass through the separator in the opposite direction and remain in solution.
As discussed above, the electrolyte used in most zinc-bromine batteries is circulated within the battery. Circulation of the electrolyte has several advantages. First it removes and externally stores bromine that is produced during charge. Thus, the active materials of the battery, which are zinc and bromine, are separated from each other. Second, circulation of the electrolyte ensures uniform zinc metal deposition and deplating during charge and discharge, respectively. Third, circulation of the electrolyte removes excess heat from the system.
While all these advantages are achieved with present circulation systems, there are still several problems that known circulation systems fail to address. One of the more significant problems is the entrapment of gas and vapor in the electrolyte as it circulates through the battery. A possible side reaction in such batteries could cause the formation of small amounts of hydrogen gas during the charging process and the hydrogen may accumulate in the stack of cells hindering the flow of electrolyte in the battery. Of less importance, but still problematic, is the entrapment of other gases and vapors that often infiltrate the battery through its pressure equalization and release valves. The presence of these gases and vapors may also hinder the flow of electrolyte.
In addition to those already noted, present electrolyte circulation systems have several other shortcomings. The circulation of electrolyte through a conventional battery is controlled through one anolyte pump/motor and one catholyte pump/motor. The catholyte pump introduces a mixture of second phase and aqueous phase electrolyte (catholyte) into the battery during discharge. Conventional technology uses one of two techniques to produce this mixture. A first technique uses a single pump inlet tube that has two branches on one end. One branch is positioned at the bottom of the reservoir tank and serves as the inlet for the dense second phase. The other branch has a pickup positioned higher in the reservoir tank and serves as the inlet for the aqueous catholyte phase. In this design, the amount of second phase introduced into the battery can not be adjusted during discharge. Also, this design requires the second phase to be circulated during both discharge and charge.
Another known design uses an outlet at the bottom of the reservoir tank and gravity to introduce the second phase into the aqueous catholyte. This technique can adjust the amount of second phase allowed into the aqueous catholyte input and even shut it off during charge. However, a major drawback to this system is that the second phase is drawn from the bottom of the tank requiring a hole to be placed at the lowest point in the tank. With a hole in this position, any compromise (puncture, break, etc.) of the outlet has the potential risk of a large electrolyte leak.
Accordingly, it would be desirable to have a system that eliminated or reduced the amount of gas and vapors entrapped in a liquid electrolyte battery. It would also be desirable to have a system where the amount of second phase circulated through the battery could be readily controlled, but with a design that would minimize the amount and effect of electrolyte leaks.